Conventional pistons of cylinder direct injection spark ignition internal combustion engines have a protruded piston head which is formed with a cavity or depression forming part of a combustion chamber, as disclosed, for example, in Japanese Patent Provisional Publication No. 8-312354. The cavity has a generally circular peripheral portion which is edge-like to form a ridgeline. The cavity is formed offset relative to the center axis of the cylinder toward an intake valve. Fuel is injected toward the cavity from a fuel injector valve in compression stroke so as to concentrically form a fuel-rich layer of stratified charge around a spark plug, thereby accomplishing stratified charge combustion. Otherwise, fuel is injected in intake stroke in accordance with an engine operating condition so as to spread the fuel in the whole combustion chamber. This forms homogeneous charge in the combustion chamber thereby accomplishing homogeneous charge combustion.
However, the above conventional piston is formed at its piston head with flat wall surfaces which are located outside the ridgeline and on the right and left sides of the cavity so as to extend parallel with an imaginary vertical plane passing through axes of a plurality of cylinders each of which is provided with the piston. Inclined wall surfaces are formed on the opposite sides of each flat wall surface so as to slope down toward the periphery of the piston head. The flat wall surfaces are positioned to traverse the combustion chamber and near the top portion of the protruded piston head. Consequently, these flat wall surfaces are deemed serving as a projected wall lying between the inside and outside of the cavity, thereby degrading combustion performance when homogeneous charge combustion is made upon fuel injection in intake stroke.
More specifically, the projected wall prevents flow of sprayed fuel from flowing between the inside and outside of the cavity. Accordingly, the sprayed fuel unavoidably tends to stay inside the cavity so that air-fuel mixture resides locally, thus preventing formation of homogeneous air-fuel mixture or charge. Additionally, the sprayed fuel is prevented from its vaporization thus making insufficient homogenization of air-fuel mixture within the combustion chamber. Furthermore, the projected wall makes the inside and outside of the cavity discontinuous, and therefore effective combustion cannot be obtained at the outside of the cavity thus degrading homogeneous charge combustion performance.
Furthermore, under the present status, the inclination angle and the like of the inclined wall surfaces have not yet been set at optimum ranges from the viewpoints of improving homogeneous charge combustion while preventing combustion performance degradation under the effect of the cavity.